1. Field of the Invention
The present invention relates to a group III nitride compound semiconductor device and a method for producing the same.
The present application is based on Japanese Patent Application No. Hei. 11-92948,which is incorporated herein by reference.
2. Description of the Related Art
A group III nitride compound semiconductor device is formed from group III nitride compound semiconductors grown on a substrate such as a sapphire substrate by a method such as an MOCVD method. On the other hand, a technique using a silicon or silicon carbide substrate inexpensive and itself electrically conductive for growing group III nitride compound semiconductor layers thereon has been examined.
In the case of a silicon substrate, however, the thermal expansion coefficient of the substrate is largely different from that of a GaN semiconductor layer. As a result, stress due to the difference between the thermal expansion coefficients may be caused in the group III nitride compound semiconductor layers in accordance with the atmospheric temperature (increased to about 1000xc2x0 C.) for growth of group III nitride compound semiconductors. When things come to the worst, cracking may occur.
The problem that stress due to the difference between the thermal expansion coefficients is caused in the group III nitride compound semiconductors is not limited to the silicon substrate. This problem may be caused as a problem which must be solved when the size of a substrate is large, regardless of the material of the substrate.
The present invention is achieved in consideration of such circumstances and the configuration thereof is as follows.
That is, a method for producing a group III nitride compound semiconductor device, comprises the steps of: forming a first environment division and a second environment division on a surface of a substrate; and laminating a plurality of group III nitride compound semiconductor layers for constituting a device on said first environment division.
According to the producing method, the group III nitride compound semiconductors for constituting a device are formed only on the first environment division of the substrate. Hence, the group III nitride compound semiconductor layers are grown in small areas on the substrate individually and separately so that the areas are not connected to one another. Hence, even in the case where the thermal expansion coefficients of the group III nitride compound semiconductor layers are different from that of the substrate, stress accumulated in the inside of the group III nitride compound semiconductor layers in each area becomes small because each region for each area of the layers is small. Hence, not only is cracking, or the like, substantially prevented from occurring in each lump of the group III nitride compound semiconductor layers but also the crystallinity of the group III nitride compound semiconductor layers themselves is improved.
Further, because each lump of the group III nitride compound semiconductor layers is so small that stress is not accumulated therein regardless of the size of the substrate, the size of the substrate can be selected at option. Hence, the productivity can be improved when the size of the substrate is selected to be large.
In the above description, the material of the substrate is not particularly limited so long as the substrate can be adapted to the group III nitride compound semiconductors. Examples of the material of such a substrate may include sapphire, silicon, silicon carbide, zinc oxide, gallium phosphide, gallium arsenide, magnesium oxide, manganese oxide, etc.
Each first environment division is a portion which is provided so that single-crystal group III nitride compound semiconductors of good crystallinity are grown thereon to thereby constitute a device. If the first environment division is in a state in which the substrate is exposed, group III nitride compound semiconductors of good crystallinity can be grown thereon. Incidentally, an undercoat layer of a metal nitride (such as TiN), a metal (such as Ti), or the like, may be formed on the first environment division in advance.
The shape of the first environment division is not particularly limited so long as a device structure can be formed thereon. Taking into account the fact that the substrate must be cut into individual devices, it is preferable to make the shape of each first environment division into a rectangle. More preferably, the shape is a square. One device may be formed in each of the first environment divisions, or each of the first environment divisions may be set to be rather large so that a plurality of devices can be formed therein. The length of each side of the rectangle is selected to be in a range of from 100 to 1000 xcexcm. If the length of each side of each first environment division is smaller than 100 xcexcm, it is impossible to form any device. If the length of each side is contrariwise larger than 1000 xcexcm, the group III nitride compound semiconductor layer grown therein becomes so large that there is a possibility that stress caused by the difference in expansion coefficient between the semiconductor layer and the substrate may be accumulated in the inside of the group III nitride compound semiconductor layer. More preferably, the length of each side of the rectangle of each first environment division is selected to be in a range of from 200 to 800 xcexcm. In an embodiment, each first environment division is shaped like a square and the length of each side of the square is selected to be 350 xcexcm which is equal to that in an existing product (light-emitting diode) sold by this applicant.
Incidentally, if corner portions of the rectangle of each first environment division are rounded off, that is, if corner portions of the rectangle are chamfered, stress to be applied to the group III nitride compound semiconductor layer is relaxed so that the crystallinity thereof is improved more greatly.
The second environment division prevents group III nitride compound semiconductors for constituting devices from being grown thereon, so that the group III nitride compound semiconductor layer lumps grown on the first environment divisions for constituting devices respectively are separated from one another by the second environment division. In other words, the group III nitride compound semiconductor layer lumps for constituting devices respectively are grown individually separately on the first environment divisions by the presence of the second environment division.
As a first mode for achieving the above description, the second environment division is made of a material such as silicon oxide, silicon nitride, or the like, on which group III nitride compound semiconductors cannot be grown. That is, the group III nitride compound semiconductor lumps grown on the first environment divisions respectively are separated from one another by a silicon oxide or silicon nitride layer formed on the substrate. The thickness of the separation layer is preferably set to be slightly larger than the designed thickness of the group III nitride compound semiconductor layer so that the group III nitride compound semiconductor lumps are not connected to one another, that is, are grown individually separately.
As another mode, the second environment division is provided so that a group III nitride compound semiconductor can be grown on the second environment division but the group III nitride compound semiconductor is made different in crystallinity (inclusive of an amorphous state) from the group III nitride compound semiconductors (for constituting devices) grown on the first environment divisions. That is, if the group III nitride compound semiconductor grown on the second environment division is inferior in crystallinity to that grown on the first environment division, internal stress due to the difference in thermal expansion coefficient from the substrate is concentrated in the structurally fragile group III nitride compound semiconductor layer grown on the second environment division so that stress can be prevented from being accumulated in the group III nitride compound semiconductor layer grown on the first environment division. In view from a different angle, if cracking is intentionally generated in the group III nitride compound semiconductor layer grown on the second environment division, the group III nitride compound semiconductor layer lumps grown on the first environment divisions are separated from one another to form small growth regions respectively so that stress can be prevented from being accumulated largely.
In other words, the above fact can be achieved if each of the first environment divisions, which is a region for the growth of the group III nitride compound semiconductor, is surrounded by the second environment division made of a material different in kind from that of the first environment division. When the first environment division is constituted by exposed portions of the substrate, a nitride compound such as BN, TiN, VN, CrN, ZrN, NbN, HfN, TaN, or the like, or an oxide compound such as TiOX, VOX, CrOX, ZrOX, TaOX, or the like, may be used as the material for constituting the second environment division in this mode.
Further, the surface of the substrate in the second environment division may be roughened. This is because any good crystal is not grown on the roughened substrate surface. Further, a level difference may be preferably provided between the second environment division having such a rough surface and the first environment division having a mirror surface suitable for the crystal growth of the group III nitride compound semiconductor. Hence, the unification of the group III nitride compound semiconductor grown on the first environment division with the group III nitride compound semiconductor grown on the second environment division is lowered so that greater independence is given to the growth of the group III nitride compound semiconductor on each of the first environment divisions.
The second environment division having such a roughened surface is formed by wet etching, scribing, half-cutting by a dicing blade, or the like.
Further, the substrate surface corresponding to the second environment division may be made amorphous by flowing ions into the substrate surface.
The width of the second environment division surrounding each first environment division is not particularly limited.
When the second environment division is constituted by a separation layer of silicon oxide, or the like, the substrate together with the separation layer may be cut into individual devices so that side faces of the group III nitride compound semiconductor layer can be protected by the remaining separation layer. In this case, the width of the separation layer needs to be larger than the width of the dicing blade. Further, when the separation layer is removed from each device, the substrate is exposed in the removed portion. When an after-process is to be applied to the exposed portion of the substrate to thereby provide another function (such as attachment of a bonding pad, or the like), it is significant from the point of view of improvement in the degree of freedom for designing the after-process that the thickness of the separation layer can be selected at option.
Each of the group III nitride compound semiconductors is represented by the general formula AlXGaYIn1-X-YN (0xe2x89xa6Xxe2x89xa61, 0xe2x89xa6Yxe2x89xa61, 0xe2x89xa6X+Yxe2x89xa61), which may further contain group III elements such as boron (B) and thallium (Tl) and in which the nitrogen (N) may be partially replaced by phosphorus (P), arsenic (As), antimony (Sb) or bismuth (Bi). Each of the group III nitride compound semiconductors may contain an optional dopant.
The method for forming the group III nitride compound semiconductor layers is not particularly limited but, for example, each of the layers may be formed by a known metal organic chemical vapor deposition method (called xe2x80x9cMOCVD methodxe2x80x9d in this specification). Alternatively, each of the layers may be formed by a known molecular beam epitaxy method (MBE method), a halide vapor phase epitaxy method (HVPE method), or the like.
When the second environment division is constituted by a separation layer of silicon oxide, or the like, the exposed substrate surface in the first environment division is in a so-called xe2x80x9cvalley floorxe2x80x9d state by the presence of the separation layer. To form group III nitride compound semiconductor layers on such a substrate surface efficiently by an MOCVD method, at least a carrier gas such as nitrogen, hydrogen, or the like, is supplied by being sprayed substantially perpendicularly onto the substrate surface. As a result, an ammonia gas and gasses of materials such as TMG, etc. can be supplied to the substrate surface efficiently. It is a matter of course that the source gasses are preferably supplied so as to be sprayed perpendicularly onto the substrate surface.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.